With the explosive growth of the Internet and other communications needs, there has developed a commensurate need for transmission systems to handle the ever increasing demand for capacity to transmit signals. Fiber optic systems have become the technology of choice for meeting this demand. Significant attention has been directed to systems which use dense wavelength division multiplexing (DWDM) to increase the number of signal channels that can be transmitted through a single optical fiber. DWDM systems rely on erbium doped fiber amplifiers (EDFA) which are pumped by semiconductor lasers. Generally, EDFAs are pumped by lasers having a wavelength of about 1480 nm or about 980 nm. There are relative advantages and disadvantages of using lasers in each of these wavelengths, which are known to those skilled in the art. EDFA's are capable of simultaneously amplifying multiple signal channels.
A schematic overview of a known generic EDFA in a DWDM system is depicted in FIG. 1. Signals enter the system through optical fiber 100 and pass through an optical isolator 120 where they are amplified in erbium doped fiber (EDF) 130 which is pumped by a plurality of semiconductor lasers 150a, 150b, 150c and 150d, each of which is fixed, or set, to a specific unique wavelength. Pumping lasers 150a-150d are coupled to EDF 130 using a conventional multiplex couplers 160 and 170. The amplified signals pass through a second optical isolator 180 and out of the system through optical fiber 190. Pumping lasers 150a-150d may be fixed at different wavelengths. For example, pumping laser 150a-150d may have output wavelengths of 1460 nm, 1470 nm, 1480 nm and 1490 nm, respectively. It will be appreciated that more than four pumping lasers may be employed in the system of FIG. 1 to further increase the number of signal channels that can be simultaneously transmitted by the system. However, the number of pumping lasers that can be effectively employed is subject to various constraints. The lasers are required to emit high power, or they are required to produce output spectrums that result in as low loss as possible when being wavelength-multiplexed with the wavelength-multiplex coupler 160. The full wavelength bandwidth of −3 dB from maximum output power is therefore required to be less than 3 nm, or still more preferably less than 2 nm in order to eliminate the coupling loss at the wavelength-multiplex coupler.
FIG. 2 shows a basic exemplary structure of a Raman amplifier. A WDM optical input signal is coupled into a Raman amplifying fiber loop 2 (a single mode fiber) by way of an input optical fiber 12 and polarization independent isolator 25. A high power beam of light, or pumping light, is generated by pumping source 1 and coupled into fiber loop 2 in the opposite direction by a WDM coupler 13 at an end of fiber 2 which is opposite to the input signal. When the high power pumping light of over 300 mW is coupled to Raman fiber loop 2, a stimulated Raman scattering phenomenon occurs in the molecules of the fiber which causes power from the high-power pumping light to be coupled the input signal light, which acts to amplify the input signal light. The transfer of power occurs if the frequency of the pumping light is about 13 THz greater than the frequency of the input signal light (which corresponds to the wavelength of the pumping light being about 100 nm shorter than the wavelength of a 1.55 μm input signal). This Raman gain has a −1 dB bandwidth of about 20 nm. In order to generate flat and wide gain band over a 80 nm band like an EDFA amplifier, a wavelength multiplexed pumping light source 1 is needed. The Raman amplifier requires a greater pump power in order to obtain the same gain as that of the EDFA amplifier. Thus, the coupling loss at the wavelength-multiplex coupler is also a important issue in this application. Therefore, in order to realize a stable Raman gain spectrum efficiently, it is important that the power level of pumping source 1 be well controlled and at the same time, the output power bandwidth of each pumping laser should be less than 3 nm, or more desirably be less than 2 nm, and the fluctuation of the center wavelength must be controlled to be less than +−1 nm. Because of the predictable nature of stimulated Raman Scattering phenomenon, a Raman amplifier can be constructed to amplify any desired wavelength so long as a pumping light source can be prepared, which is an advantage over the EDFA amplifier.
The present invention is directed towards semiconductor lasers that can be employed by pumping source 1. As a brief background, pumping source 1 comprises semiconductor lasers 31, 32, 33, and 34 of Fabry-Perot type, wavelength stabilizing fiber gratings 51, 52, 53, and 54, polarization couplers (polarization beam combiner) 61 and 61, and a WDM coupler 11. The fiber gratings 51, 52, 53, and 54 are wavelength-selective reflectors which set the center wavelengths of lasers 31, 32, 33, and 34, respectively. Gratings 51, 52 set the center wavelengths of lasers 31, 32 to a first wavelength λ1, and gratings 53, 54 set the center wavelengths of lasers 33, 34 to a second wavelength λ2. The difference between λ1 and λ2 is between 6 nm and 35 nm, and additional sets of lasers and gratings at different wavelengths may also be added to pumping source 1. Light outputs from lasers 31, 32, 33, 34 are polarization-multiplexed by the polarization coupler 6 for each wavelength λ1, λ2, and output lights from the polarization coupler 6 are combined by the WDM coupler 11 to obtain the output light of pumping source 1. Polarization maintaining fibers 17 are connected between the semiconductor lasers 3 and the polarization coupler 6 to obtain two pumping lights having different polarization planes.
A portion of the output light is coupled by a branching coupler 14 and analyzed by a monitoring and control circuit 15, which determines the amount of amplification that is occurring and generates a feedback control signal to pumping source 1 which ensures consistent amplification (gain).
While FIG. 1 shows the basic construction of an EDFA amplifier and FIG. 2 shows the basic construction of a Raman amplifier, there are many challenges remaining for improving the performance and efficiency of the system, and it is believed that nearly every component of the system can be improved. Among the challenges addressed by the present invention is the need for a higher power pumping laser which has a center wavelength and a level of power that are well-controlled, and which has a very narrow bandwidth output that can be wavelength multiplexed with other such lasers, each typically fixed at different center wavelengths.
Greater power from the pumping lasers enables a repeater in a DWDM system, which typically comprises an EDFA or Raman Amplifier, to amplify the incoming signals to a greater degree, which enables the distance between repeaters to be increased. The latter enables one to reduce the number of repeaters in the system, thereby lowing the cost of the system and increasing the reliability of the system. Greater power also enables the EDFA to amplify more signal channels, and thereby enables the DWDM system to carry more signal channels.
Stable and narrowly confined power from the pumping lasers enables a low loss multiplexing of the individual pumping light with wavelength multiplexing couplers, thereby enabling a multiplexed power to be greater. A stable center wavelength and a well-controlled level of power also enables a Raman gain produced by the pumping light to be stable, thereby preventing associated noise from being modulated onto the input signal.
Conventional approaches to realize a high output power performance of a single laser chip by itself, and the problems to be solved are described below. A conventional semiconductor laser chip (LD) shares some common features with laser chips according to the present invention; for the sake of keeping the number of figures low, we will describe a conventional LD with reference to an exemplary LD 210 according to the present invention which is shown in FIGS. 3 and 4, with FIG. 4 being a cross-sectional view of the device of FIG. 3 across view lines 4—4. The LD 210 shown in FIG. 4 is a buried heterostructure (BH) type Fabry-Pérot laser which is fabricated using standard processes for semiconductor laser device fabrication. In FIG. 4, multi-quantum well (MQW) structure is conventionally adopted for active layer 450. The active layer 450, upper and lower graded-index separate confinement hetero-structure (GRIN-SCH) layers, 440 and 460, respectively, are formed in a limited spatial region within laser device 210. Adjacent to this structure, current blocking layers 21 and 22, which may be formed of p-InP and n-InP respectively, confine the current flow so the current from electrodes 330 and 410 is injected into active layer 450. After the laser device is cleaved in the cavity length L, a low-reflectivity film 310 is formed on the “front” facet of the device, and a high-reflectivity (HR) film 320 is formed on the “rear” facet, opposite the front facet. Low-reflectivity film 310 is also referred to as anti-reflective(AR) film 310. The features thus far described are common to both the conventional LD and an LD according to the present invention. In a typical prior art laser for high power pumping applications, the cavity length L and the reflectivity of the front facet (as realized with the low-reflectivity film) for the practical high power laser have been chosen to be less than 900 μm and greater than or equal to 4%, respectively.
In realizing high power LDs, the advantages of adopting 1) strained quantum well structure for the active layer 450 and 2) longer cavity length L has been separately known in the prior art in terms of improving intrinsic performances such as gain performances and of improving the thermal conductance of the chip. However, it was also known that the maximum output power Pmax tends to saturate when a cavity length exceeds 1000 μm, with the front-facet reflectivity kept to the same level as that used in the conventional prior art laser chip (greater than or equal to 4%). (In this specification, the term “maximum output power” or “Pmax” refers to the highest optical output power observed when the injection current is increased. The maximum occurs partly due to the drop in quantum efficiency caused by the rise in temperature in the active layer as the current is increased.) For this reason, there has been little motivation to use cavity lengths greater than about 700 μm to 900 μm. Thus, the possibility of improving the maximum output power Pmax by combining the above two approaches 1) and 2) at a cavity length beyond 1000 μm had not been seriously studied or considered as being feasible. Particularly, the optimum range in the combination of cavity length of the laser chip and front-facet reflectivity of the chip had not been studied as a practical matter.
In addition to high optical output power, it is important for each individual laser module in a WDM pumping module to have a fixed wavelength output which is independent of the drive current and the environmental (i.e., temperature) conditions. A particular problem with semiconductor pumping laser devices used in fiber optic communications systems is that they tend to have relatively broad output wavelength spectrum which varies with drive current and temperature.
One approach for a stabilized, narrow wavelength band pumping laser has been to use a fiber Bragg grating (FBG) external to the semiconductor laser device which is optically coupled to the laser output and which forms external reflection surfaces of the laser device. The fiber Bragg grating may be fabricated to have a relatively narrow reflectivity bandwidth, such that the output spectrum of the laser is kept within this narrow band. One prior approach has set the reflection band width ΔλFBG of the FBG to be larger than the twice the longitudinal mode (FP mode) spacing ΔλFP of the semiconductor laser chip in order to suppress kinks that otherwise appear in the output power vs. injection current diagram. Another prior art approach has used a FBG bandwidth ΔλFBG of 2 nm to 5 nm in the pumping laser module at a wavelength band around 1480 nm. However, those prior art approaches are based on experiments done on LD's with cavity lengths less than or equal to 900 μm.
As a consequence of their experimental effort to improve the maximum output power Pmax by utilizing longer cavity LD's, the inventors have surprisingly found that the kink problem still exists in the FBG-coupled pumping laser module when LD chips with cavity lengths longer than 1000 μm are used, even if the bandwidth of the FBG includes a plurality of FP modes. Experiments done on a laser with a cavity length of L=1300 μm and a lasing wavelength at 1480 μm (ΔλFP=0.24 nm) coupled to a 4 nm bandwidth (ΔλFBG) FBG, which corresponds to as many as 16.7 FP-mode spacings, revealed kinks that appeared in the output power vs. injection current curve, as shown in FIG. 9. This figure shows the output power POUT as a function of the diode current ID (Left Axis), and the derivative, or slope, of the POUT curve, which identified as dPOUT/dID (Right Axis). The presence of the kinks is most easily seen in the derivative curve as sharp changes in slope direction. The measurement shows that the prior art teachings are not successfully applicable to FBG-coupled LD's with cavity length of 1000 μm or longer.
Furthermore, the prior art does not teach constructing the reflectivity characteristics of the low-reflectivity film to provide maximum output power as combined with the reflectivity characteristics of the FBG, or that there is any beneficial relationship between these two reflectivity characteristics, especially when long cavity LD's are used.